Next story in Science

SAN FRANCISCO — More than 150 years ago, a fault ringing the
Caribbean shook half the Atlantic, including New York City, with
a mega-earthquake. The quake rivaled those that have struck
Indonesia in recent years, geologists reported last week at the
annual meeting of the American Geophysical Union.

The Caribbean's beautiful tropical islands and coral reefs rise
above a complex junction of four major tectonic plates. Many of
the islands sit above a subduction zone, where two plates meet
and one slides haltingly under the other, down into the Earth's
mantle. The Dec. 26, 2004, Sumatra, Indonesia, earthquake, a
subduction zone earthquake that generated deadly tsunamis, has
galvanized scientific interest in potential quake hazards from
the
Caribbean's similar earthquake-producing faults.

The Feb. 8, 1843, Lesser Antilles earthquake was in many ways
remarkably similar to the magnitude-8.7 earthquake that struck
Sumatra just one year later, in 2005, researchers reported at the
meeting. [ The
10 Biggest Earthquakes in History ]

Historical sleuthing by Francois Beauducel and Nathalie Feuillet
of the Paris Institute of Earth Physics upwardly revised the
temblor's
estimate of magnitude to 8.5 (from a previous estimate of
7.8).

Maps from the French national marine service revealed that many
palm-covered islets in the bay at Pointe-à-Pitre, the biggest
city on the island of Guadeloupe, disappeared between 1820 and
1869, Feuillet told OurAmazingPlanet. The islets likely subsided,
or dropped below sea level. The stress of the two plates stuck
together makes the earth's crust flex and warp. After the
earthquake, the deformed crust rebounds in some areas and drops
down in others.

Combining the 19th-century records of such effects with modern
earthquake models helped Beauducel and Feuillet pin down both the
quake's magnitude and the location of the fault rupture, the spot
where the subduction zone tore apart.

"The only way to explain the subsidence of the islands is to have
a rupture … in the very deep part of the subduction zone, between
40 and 60 km (25 to 40 miles) depth," Feuillet said. Such as
depth is comparable to that of the 2005 Sumatra quake, the
researchers said.

The quake was felt up and down the East Coast, including in New
York City, Washington, D.C., Raleigh, N.C., and Charleston, S.C.,
said Susan Hough of the U.S. Geological Survey. Hough also
unearthed reports of shaking at three locations in South America,
she said.

But, just as with the
2005 Sumatra quake, there was no giant wave on Feb. 8, 1843.
Reports describe a 4-foot (1.2 m) wave in Antigua, but no
significant tsunami arose, Feuillet said. Even so, several
thousand people died in Pointe-à-Pitre from fires and damage
caused by the severe shaking.